On the description of noise in quantum systems

N. G. van Kampen

Andrew Kolmer Forbes

Nonclassicality in quantum sensors can improve sensitivity, but often increases susceptibility to noise. Thus, modeling physically relevant noise sources and analyzing their effect on quantum metrology are both of importance to the field of quantum sensing. In this dissertation, I demonstrate that local noise sources, which are present in almost all many-spin systems, can be tractably modeled when assuming permutation symmetry of the noise, and we show that many common local noise sources can be mapped to a Fokker-Planck equation on quantum phase space. We apply this description of noise to study quantum sensing using noisy probe states and establish a previously unknown connection between loss of Fisher information for noisy states and discontinuities in the Fisher information as a function of a continuous family of measurements. Finally, we extend the phase space description of local noise by developing a new spin Wigner function, which we call the solid spin Wigner function. We develop this new Wigner function by noting that the permutationally symmetric operator space used to describe symmetric local noise resembles the symmetric subspace of an ensemble of SU(3) particles. Using this, it is possible to embed the SU(2) group structure of the spin ensemble within this larger group, and describe the system using an SU(3) Wigner function.

W. H. Louisell

Ivan Medina, F. L. Semião

We apply reverse engineering to find electromagnetic pulses that allow for the control of populations in quantum systems under dephasing and thermal noises. In particular, we discuss two-level systems given their importance in the description of several molecular systems as well as quantum computing. Such an investigation naturally finds applications in a multitude of physical situations involving the control of quantum systems. We present an analytical description of the pulse which solves a constrained dynamics where the initial and final populations are fixed a priori. This constrained dynamics is sometimes impossible, and we precisely spot the conditions for that. One of our results is the presentation of analytical conditions for the establishment of steady states for finite coherence in the presence of noise. This might naturally find applications in quantum memories.

Satyabrata Bhattacharya, Pinaki Chaudhury, Sudip Chattopadhyay +1 more

A system-reservoir nonlinear coupling model is proposed for a quantum system when the associated bath is not in thermal equilibrium but is modulated by an external colored noise, to present a microscopic approach to quantum state-dependent diffusion and multiplicative noise in terms of a quantum Langevin description. Consequently, the Fokker-Planck equation in position space, valid in the overdamped limit, for multiplicative colored noise is constructed to explore the possibility of observing a quantum current and dependence of the current on various parameters of external noise is examined.

Ahsan Nazir